Conventionally, exhaust emission control devices with diesel particulate filters (Hereinafter abbreviated as “DPF”), which are provided for collecting particulate matter (Hereinafter abbreviated as “PM”) included in the exhaust gas, are equipped in diesel engines. For example, the said DPF is made of ceramics forming a honeycomb structural porous media. PM is collected when exhaust gas passes through the DPF, and as a result, the exhaust gas is defecated.
In the above mentioned diesel engines, PM accumulates inside the DPF on the process of cleaning up exhaust gas by means of the exhaust emission control device. This will induce functional depression of the exhaust emission control device. Moreover, increase of exhaust resistance or the like will cause bad effect on the diesel engine. Because of this, control for regeneration of the DPF is implemented so as to remove PM accumulated in the DPF at regular intervals. The said regeneration of the DPF is implemented when estimated amount of accumulated PM reaches a prescribed threshold value. The amount of accumulated PM is estimated on the basis of a differential pressure between the upstream side and the downstream side of the DPF in the flow direction of exhaust gas (Hereinafter abbreviated as the “differential pressure between before and after the DPF”).
However, not only combustible ingredients such as soot which can be oxidatively removed, but also ash derived from fuel addition agent or lubricating oil addition agent or the like is included in PM. Since the said ash cannot be oxidatively removed, progress of accumulation of PM will cause big divergence between the estimated amount of accumulated PM which is estimated from the differential pressure between before and after the DPF and the real amount of accumulated PM. In that case, regeneration of the DPF cannot be appropriately performed.
To solve such problems, an art for regenerating the DPF suitably is disclosed in the Patent Literature 1. In this art, amount of accumulated ash in the DPF is estimated on the basis of an operating time of the diesel engine. And the estimated amount of accumulated PM which is calculated on the basis of the differential pressure between before and after the DPF soon after implementing regeneration of the DPF is considered as an amount of accumulated ash. In this way, amount of accumulated ash with high accuracy is obtained.
However, in the art of the Patent Literature 1, PM is oxidatively removed by means of an additional injection or the like only when the amount of accumulated PM estimated from the differential pressure between before and after the DPF exceeds the prescribed threshold value. That is to say, operation conditions of the diesel engine or the temperature of exhaust gas etc. are not considered. For this reason, it is disadvantageous because regeneration of the DPF cannot be performed efficiently. It is also disadvantageous because of leading to problems such as dilution of lubricating oil caused by an increase in frequency of regeneration by the additional injection, or bad fuel consumption.